Although gender disparities in cardiac disease are recognized, the mechanisms through which pre-menopausal females are protected have not been fully elucidated. Cardiac disease incidence in females increases post-menopause, suggesting a role for estrogen in pre-menopausal cardioprotection. However, clinical trials found no beneficial cardiovascular outcomes from hormone replacement therapy, indicating a better mechanistic understanding is needed. Thus the goal of this study is to understand the mechanism responsible for the male-female differences in ischemia-reperfusion injury and cardioprotection. Dilated idiopathic cardiomyopathy (DCM) is one of the most common types of cardiomyopathy. Sex differences in hypertrophy are well known. We examine the role of sex differences in S-nitrosylation and oxidation from explanted DCM and non-failing donor male and female human hearts. Redox-based resin-assisted capture for oxidation (Ox-RAC) and SNO (SNO-RAC) proteomic analysis were used to measure protein oxidation and SNO respectively. In addition, a 2D-DIGE using DyLight-maleimide sulfhydryl-reactive fuors was used to identify the SNO proteins. Protein oxidation increased in DCM biopsies in comparison with healthy donors. Interestingly we did not find a consistent decrease in SNO in failing hearts; we found that some proteins showed an increase in SNO and others a decrease and there were sex differences in the response. We found 10 proteins with a significant decrease in SNO and 4 proteins with an increase in SNO in failing female hearts. Comparing non-failing and failing male hearts we found 9 proteins with a significant decrease and 12 proteins with a significant increase. We also found an increase in S-glutathiolation of eNOS in failing female versus male hearts, suggesting an increase in uncoupled NOS in females. These findings highlight the importance of the nitroso/redox signaling in both physiological and pathological conditions, suggesting a potential target to treat HF.It has been proposed that an increase in oxidative stress in heart failure leads to a decrease in nitric oxide signaling, leading to impaired nitroso-redox signaling. To test this hypothesis we investigated the occurrence of protein S-nitrosylation (SNO) and protein oxidation in biopsies from explanted DCM and non-failing donor male and female human hearts. A modied biotin switch method using DyLight-maleimide sulfhydryl-reactive fluors was used to identify the SNO proteins and a resin-assisted capture Ox-RAC was used to measure protein oxidation. We found that oxidative stress rises in DCM human biopsies in comparison with healthy donors. DyLight-maleimide gel electrophoresis showed that females have an increase in protein SNO compared with males at baseline. Interestingly many of the proteins that showed an increase in SNO in females are mitochondrial proteins. Because there are sex differences in SNO at baseline, we examined changes in SNO in heart failure as a function of sex and found sex differences in this response. Steroid hormone receptors, ER-alpha and ER-beta, classically function as transcription factors regulating gene expression. Recent data indicate that estrogen can also elicit effects by binding to estrogen receptors (ER-alpha, ER-beta; and GPR30) at the plasma membrane and initiate kinase signaling. We investigate the hypothesis that that non-nuclear ER activation reduces cardiac I-R injury in mice. We used an estrogen-dendrimer conjugate (EDC), which has been demonstrated in mice to be a non-nuclear selective ER modulator. We treated ovariectomized wild type mice with EDC, estradiol or dendrimer control for two weeks using Alzet osmotic minipumps. Isolated hearts were perfused in the Langendorff model and subjected to 30 minutes ischemia and 90 minutes reperfusion. Two weeks of treatment with estradiol significantly decreased infarct size and improved post-ischemic contractile dysfunction (40.42.5% vs. 62.95.8% for infarct and 44.74.0% vs. 27.02.7% for post-ischemic functional recovery). Similarly, EDC treatment significantly decreased infarct size (40.93.6% for EDC vs 63.84.7% vehicle) and increased post-ischemic functional recovery (48.83.0% EDC vs. 28.62.5% vehicle) compared to vehicle-treated hearts. Uterine weight was significantly increased by estrogen treatment but not by EDC. To distinguish whether protection was mediated by GPER or classic ER we used ICI 182,780, which is an antagonist of ER-alpha and ER-beta, but an agonist of GPER. ICI 182,780 significantly blocked the EDC-mediated cardioprotection. To better understand which ER was involved in cardioprotection, we generated cardiac-specific ER-alpha; knockout mice. In these mice, EDC treatment significantly decreased infarct size (20.11.9% vs. 51.27.8% dendrimer) and improved functional recovery (65.84.2% vs. 36.85.2% dendrimer) compared to the vehicle-treated ER-alpha; knockout mice. These results indicate that EDC is effective in providing cardioprotection during ischemia-reperfusion injury in mice, indicating that non-nuclear ER actions play a major role in this protection. Moreover, non-nuclear ERs; and/or GPR30 (or ERs; in other cell types) are likely candidates to mediate cardioprotection during ischemia-reperfusion injury. Thus, EDC could be utilized clinically to provide cardiovascular benefit without the classical steroid hormone side effect, such as uterine and breast cancer.